1. Field of the Invention
The present invention relates to an optical transmission module having a light emitting device that converts an electric signal into an optical signal.
2. Description of the Related Art
In optical communication systems using light as an optical transmission means, optical transmission modules having a light emitting device for converting an electric signal into an optical signal are used as transmission equipment for transmitting an optical signal through an optical transmission line such as an optical fiber (see, for example, Japanese Unexamined Patent Application Publication No. 11-109184).
FIG. 9 is a partially cutaway side view showing the configuration of an example of a conventional optical transmission module. An optical transmission module 6 is a coaxial optical module in which a laser diode 65 serving as a light emitting device and the like are placed in a housing 60 such as a metal package. Inside the housing 60 of the optical transmission module 6, a ferrule 61, a converging lens 63, and the laser diode 65 are arranged so that their optical axes are aligned with one another. An optical signal emitted from the laser diode 65 enters an optical fiber 62, which extends in the ferrule 61, via the converging lens 63, and is then output from the optical fiber 62 to the outside. A photodiode 66 is placed on the backward side (lower side) of the laser diode 65. The photodiode 66 detects backward light from the laser diode 65 in order to monitor the driving state of the laser diode 65.
FIG. 10 is a sectional side view of another conventional optical transmission module. FIG. 11 is a top plan view of the optical transmission module shown in FIG. 10. An optical transmission module 7 is a surface-mounted optical module in which a laser diode 80 and an optical waveguide element 85 having an optical waveguide 86 that transmits and outputs an optical signal from the laser diode 80 are placed on a substrate 70.
In the optical transmission module 7, a ferrule 90, the optical waveguide element 85 having the optical waveguide 86, and the laser diode 80 are arranged so that their optical axes are aligned with one another. An optical signal emitted from the laser diode 80 enters an optical fiber 91, which extends in the ferrule 90, via the optical waveguide 86, and is then output from the optical fiber 91 to the outside. A waveguide type photodiode 81 for monitoring backward light is placed on the backward side of the laser diode 80. In the coaxial optical transmission module 6 shown in FIG. 9, the size is increased because of the three-dimensional structure, and this limits cost reduction. In contrast, in the surface-mounted optical transmission module 7 shown in FIGS. 10 and 11, the size and cost of the module can be reduced.
In an optical transmission module, a driving device, such as a driving IC, must be provided to control the driving of a light emitting device that converts an electric signal into an optical signal and outputs the optical signal so as to be transmitted by an optical communication system. In the coaxial optical transmission module 6 shown in FIG. 9, the ferrule 61 and optical elements such as the lens 63, etc. are placed on downstream side (upper side) of the laser diode 65, and the photodiode 66 and a metal base of the housing 60 are located on the backward side (lower side).
In the optical transmission module 6 having such a configuration, a driving device is placed outside the housing 60, or apart from the laser diode 65 inside the housing 60. In this case, the connection length between the laser diode 65 and the driving device is long, and this increases the size of the module. Moreover, it is difficult to drive the laser diode at high speed, because of the impedance of a wire between the laser diode 65 and the driving device.
In the case where a driving device is provided to the surface-mounted optical transmission module 7 shown in FIGS. 10 and 11, after the module 7 is mounted on a wiring board 95, as shown in FIG. 12, the module 7 is connected through wires 96 to a driving device 97 placed on the wiring board 95. The driving device 97 is also connected to a control IC 98. In such a configuration, the connection length between the laser diode 80 of the optical transmission module 7 and the driving device 97 is also long. Moreover, since the driving device 97 is provided outside the optical transmission module 7, the entire transmission equipment is increased in size.
In recent years, optical transmission systems have been required to increase the transmission data-rate at which optical signals are transmitted for communication, for example, from 1 Gbps to 5 Gbps. Optical transmission modules for transmitting optical signals are similarly required to have a higher transmission data-rate. In addition, in order to achieve optical communication systems capable of efficiently conducting multiple communications, size reduction of optical modules, such as an optical transmission module, is necessary.